# JS rounds off and retains two decimal places without introducing a third-party library

Time：2020-9-15

### 1. toFixed()

The tofixed() method rounds number to a number of specified decimal places

Most people’s first reaction is to use ToFixed (), but there is a hole in tofixed() method

But the rounding rules are different from the rules in mathematics, which useBanker rounding rules

The essence of the so-called banker rounding method is a kind ofThe method of rounding off and taking even

In short:Round six into five consideration, after five non-zero into one, after five zero to see odd even, five before even should be discarded, five before odd to enter one

``````>(3.61). ToFixed (1) // rounding
'3.6'
>(3.69). ToFixed (1) // six input
'3.7'
>(3.651). ToFixed (1) // consider five, non-zero after five, advance
'3.7'
>(3.65). ToFixed (1) // consider five, zero after five, even before five, round off
'3.6'
>(3.75). ToFixed (1) // consider five, zero after five, odd before five
'3.8'``````

Obviously, the way we deal with the data is different from the way we deal with the data.

### 2. Round method

The round () method rounds a number to the nearest integer. It’s the same rule of choice in mathematics.

``Math.round(X * 100) / 100``

Most articles on the Internet recommend this method, but there are some problems in JavaScript about the accuracy of floating-point calculation.

``````//This doesn't seem to be a problem
5.1 * 100
> 509.99999999999994
Math.round(5.1 * 100) / 100
> 5.1

//Is there something wrong with this
316.655 * 100
> 31065.499999999996
Math.round(316.655 * 100) / 100
> 316.65``````

Here are the methods used in my project,

``````function fomatFloat(num, n) {
var f = parseFloat(num);

if (isNaN(f)) {
return false;
}
f =  Math.ceil (num *  Math.pow (10, n)) /  Math.pow (10, n); // N power round up
var s = f.toString();
var rs = s.indexOf('.');
//If it is an integer, add a decimal point and add 0
if (rs < 0) {
rs = s.length;
s += '.';
}
while (s.length <= rs + n) {
s += '0';
}
return s;
}``````

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